The present invention relates to a method for preparing cross-linked hydrogels which are tough, rubbery and comparatively very strong, considering their content of water at swelling equilibrium. Their tensile strength is, at the same content of water, higher by 1 - 2 orders than that of usual known hydrogels such as sparingly cross-linked hydroxyethyl methacrylate polymers. High strength, elasticity and permeability to water and solutes, in conjunction with physiological inertness, of the instant hydrogels makes possible new useful applications in surgery and in various fields of industry. The outstanding physical properties of the new hydrogels are due to their particular structure of two different sorts of macromolecular networks, one formed by polyacrylonitrile domains and possessing physical bonds (dipoles, hydrogen bonds etc.), the other formed by covalent bonds caused by chain transfer onto the monomer during the polymerization. The last mentioned covalent network is thinner than the physical network mentioned in the preceding sentence, and is, moreover, formed by considerably long chain bridges. As a result, gels with a comparatively low degree of hydrolysis and having a swelling capacity in water not exceeding about 50 % by weight can be partially oriented by stretching, particularly if the stretching is carried out at temperatures higher than about 70.degree. C and the stretched hydrogel is cooled to room temperature in stretched condition. The stretching degree depends upon the swelling capacity (which is directly proportional to the degree of hydrolysis) and upon the density of the covalent network, being inversely proportional in both cases. The partially oriented hydrogels maintain a considerable elasticity and their deformation takes place so evenly that the stress-strain curve approaches a straight line. More hydrolyzed gels cannot be oriented permanently at room temperature, and their behavior is more like that of vulcanized rubbers. With an increasing degree of hydrolysis the strength decreases, due to decreasing content of dry substance (the solid remainder after evaporating the water) in swelled condition. As could be expected, at high degrees of swelling, the cross-linking affects adversely the structural strength, particularly in the case of more tightly cross-linked gels.
As can be seen, skillful combination of the cross-linking degree with the degree of partial hydrolysis makes it possible to control the physical characteristics of the instant hydrogels so that they are suitable for various end uses.
Hydrogels swelled in aqueous liquids incapable of dissolving polyacrylonitrile possess the two above mentioned kinds of macromolecular networks simultaneously. The two networks can be differentiated by swelling the hydrogel in a liquid capable of dissolving polyacrylonitrile. Then, the first mentioned sort of physically cross-linked network is dissolved and only the covalent bonds remain intact. A suitable solvent for polyacrylonitrile is e.g. dimethyl sulfoxide. The hydrogen swells considerably therein, because the solvent is solvating also the acrylamide units. The swelling capacity depends then on the density of the covalent network. If the covalent network is thin and the solvent is very effective (such as a 60 % sodium rhodanide solution in water), the covalent bonds can be partially destroyed by osmotic pressure and the hydrogel collapses and partially dissolves. In such extreme cases, the presence of the covalent network must be proved by the use of finer methods, e.g. mechanometric analysis.
Moderate or decreased temperature during the hydrolysis is obviously necessary to avoid uncontrollable oxidation of the polymer by nitric acid, such oxidation taking place slowly above about 45.degree. C and rapidly above about 50.degree. C. The same holds naturally for the partial hydrolysis. The temperature up to which the gel swelled by concentrated nitric acid can be heated without danger of an uncontrollable decomposition depends on the degree of purity, the impurities being more easily oxidized.
When polymerizing acrylonitrile in nitric acid to obtain spinning solutions of non-crosslinked polyacrylonitrile, stabilizers for preventing oxidation can be added, e.g. urea. This is, however, practically impossible in the present process, because the stabilizers act simultaneously as chain transfer agents. This side-effect is desirable when preparing solutions of polyacrylonitrile since the linear chains are not only shortened, but simultaneously their length becomes more uniform. As said above, any chain transfer agents are to be excluded so that the chain transfer affects the monomer only. Thus, the absence of stabilizers necessitates the use of lower temperatures to avoid the danger of rapid or even explosive decomposition, coupled with the liberation of poisonous nitrous gases.
It has been found, however, that lower temperatures, namely room or slightly decreased temperatures, are advantageous also for other reasons. The chain transfer onto the monomer is preferred over the less desirable chain transfer onto the polymer, the latter being more temperature-dependent than the former. Moreover, the heat of polymerization is more easily dissipated at lower temperatures. A further advantageous effect of decreased temperatures is an increased length of polymer segments between the points of cross-linking, as well as an increased length of the crosslinks and branches, all resulting in better physical characteristics of the hydrogel. All of this contributes to the explanation of the unusual strength and elasticity of the instant hydrogels.
An object of the invention is therefore a new method for manufacturing highly swellable and strong, elastic hydrogels, well tolerated by living tissues and permeable to water and low-molecular solutes, and insoluble in any solvent.
Another object of the invention is to provide a new method for manufacturing hydrogels of the above disclosed kind in an economical, reliable and reproducible way.